The present invention relates to an improved sealing apparatus and method. More specifically, the present invention relates to an apparatus and method for sealing the interface between two channels located in communication with each other in an injection mold, thereby transporting high pressure, heated molten resin towards mold cavities in a cycling operation without leakage.
Avoiding leakage of the hot and pressurized molten resin material represents a major challenge when designing an injection molding machine or an injection mold. For example, a critical leakage area in an injection molding machine is between the machine""s injection nozzle and the mold""s sprue bushing. Leakage commonly appears in an injection molding machine between the manifold and the mold nozzles or at the interface between the mold nozzle and the mold cavity.
Injection molding manifolds are usually made of a massive runner block communicating with the injection nozzles located adjacent the mold cavities. Leakage of the molten resin material at the interface of the runner block and the injection nozzle, for example, represents a major problem due to the high pressure of the heated, flowing molten resin and the relative differential thermal expansion of the materials which makes the runner block slide laterally with respect to the injection nozzles. Sealing of the fluid interface between the internal channels located in the runner block or manifold and the injection nozzle represents, therefore, a significant design problem, especially taking into account that the injection process must be stopped if leakage occurs.
The prior art teaches several sealing methods and elements that have been developed, but these do not satisfactorily solve the leakage problem especially at the interface between the manifold and the mold nozzle. In addition, the prior art allows large compressive forces to be generated within the injection manifolds, requiring the use of thick steel plates and numerous structural fasteners.
Known design concepts in injection molds use a small pre-load in cold condition between the manifold and the nozzle. This small pre-load accompanies the inherent thermal expansion of the manifold to provide sufficient compression between the parts to maintain sealing between the manifold and nozzle or between other channels in the system during operation. However, while too little compression results in plastic leakage, extreme compression causes either permanent setting of the manifold steel or damage to the nozzle housing. During operation, forces between the manifold and nozzle can be in excess of 10,000-14,000 pounds for each nozzle. These large forces require the use of massive blocks of steel and numerous high-strength fastners within an injection mold machine. In addition to this, the prolonged and cyclic injection molding operation will reduce the effectiveness of the pre-load, thus increasing the likelihood of leakage.
Several improvements to these design concepts have been developed that use different methods and means to prevent leakage of the plastic resin.
U.S. Pat. No. 3,849,048 to Bielfeldt (incorporated herein by reference) shows a hydraulically actuated back-up pad that takes up the cold clearance to prevent leakage. This piston acts like a spring. Inside the housing is a second hydraulic piston which drives the valve stem. The nozzle body is threaded into the manifold insert and therefore thermally expands laterally when the manifold expands. The close proximity of flammable hydraulic oil to the heated manifold means that there is a great risk of fire with this design after the seals have worn.
U.S. Pat. No. 3,716,318 to Erik (incorporated herein by reference) shows a combined nozzle/manifold bushing piece which is inserted through the manifold from the underside and is retained by a threaded back-up pad. This construction is also disadvantageous in that the nozzle assembly must travel laterally with the manifold as it thermally expands.
U.S. Pat. No. 3,252,184 to Ninneman (incorporated herein by reference) shows a manifold bushing piece inserted through the manifold and butted against the spigotted end of the nozzle body. Because the nozzle body is spigotted to the manifold, it must travel laterally when the manifold thermally expands.
U.S. Pat. No. 3,023,458 to Seymour (incorporated herein by reference) illustrates a one piece manifold bushing and nozzle body inserted through the manifold. The valve stem is closed with a spring and opened via injection pressure. The nozzle end of the bushing appears to be located in a recess in the mold cavity plate and clearly cannot accommodate lateral thermal expansion of the manifold plate with respect to the cavity plate. In effect, bending occurs which would tend to cause the valve stem to bind.
U.S. Pat. No. 5,896,640 to Lazinski, et al. (incorporated herein by reference) teaches an annular shaped thermal expansion element to provide improved sealing at the interface between the manifold and the nozzle body. This annular shaped element comprises an angular, spring-like radial surface that interfaces with the underside of the nozzle body shoulder, thereby enhancing the sealing pressure profile at the mating surfaces. This apparatus, while providing an improved pressure profile for sealing adjacent the channel, still produces large compressive forces that require the use of large manifold structures.
U.S. Pat. No. 4,588,367 to Schad (incorporated herein by reference) teaches a thermal expansion element to seal the flow of resin through the interface between the manifold channel and injection nozzle, or a thermal expansion element with an undercut to give it additional elastic sealing properties, or a thermal expansion element with a spring element to enhance the sealing properties by adding an elastic feature. The thermal expansion element allows relative movement between the manifold, thermal expansion element and nozzle.
U.S. Pat. No. 5,374,182 to Gessner (incorporated herein by reference) uses a spring which deflects as the nozzle body and the air back-up pad expand due to the increase in temperature. The sealing device uses Belleville style disc springs assembled on an insulating sleeve. As the manifold heats up the disc spring package absorbs the thermal expansion and prevents over-stressing the nozzle housing or setting of the manifold""s plate steel. This design provides a superior anti-leaking solution in many situations where the injection pressure remains relatively small. The disc spring system of the ""182 patent loads the flange of the nozzle housing in a purely axial direction perpendicular to the interface surface between the nozzle and the manifold plate. By providing an axial sealing force, the profile of the sealing stress shows a significant decrease towards the melt channel relative to the peak achieved at the point of contact between the spring and the nozzle. In the event that the injection pressure reaches higher valves this improved design does not effectively prevent leakage of the molten plastic resin outside the passageway.
U.S. Pat. No. 5,507,637 to Schad et al. (incorporated herein by reference) teaches a sealing clamp ring attached to the manifold and surrounding the nozzle housing that prevents leakage of the resin at the interface between the manifold and the nozzle. A certain lateral clearance that remains between the clamp ring and the nozzle allows the manifold and the clamp ring to slide laterally without affecting the alignment of the nozzle tip with respect the mold gate.
The design concepts of the cited patents represent a significant advance and use a sliding interface between the manifold and the nozzle housing. As the manifold heats and expands it also slides across the nozzle housings which are held in the cavity plate counter bores. This allows the nozzle tip location to be maintained in proper alignment with the mold gate, independent of the temperature of the manifold. However, in a sliding interface between the nozzle and the manifold it is difficult to fully seal the interface between the channels of the two parts using the sealing means disclosed in these references and one cannot easily achieve a sealing stress distribution which has its peak adjacent the channels. In addition, all of these prior art examples rely on the use of high forces, generated by their spring(s) or spring-like structure to ensure the interface sealing is effective over a wide range of temperatures, thereby eliminating the risk of leakage at the interface. These large forces mean the structure of the mold and the manifold must be designed to handle loads between 10,000-14,000 lbs per nozzle.
An apparatus of the present invention for injecting plastic material comprises a manifold having a melt channel and a flat sealing surface, and a nozzle assembly seated directly against the flat sealing surface. The nozzle assembly includes a nozzle body having an axial channel aligned, in use, with the melt channel in the manifold for communicating a flow of material therein. The nozzle body has a non-flat sealing surface adjacent the flat sealing surface, thereby forming a sealing interface to seal the nozzle body with the manifold. The flat sealing surface may be on an end of a bushing mounted into the manifold. The non-flat surface may have a conical profile, preferably defined by an angle less than one degree, and preferably between 0.2 to 0.4 degrees, from a plane parallel to the flat sealing surface. The non-flat surface may have a spherical profile, preferably having a radius between 350 mm and 4000 mm.
The apparatus may also include a compressive force regulator for limiting compressive forces adjacent the melt channel and the axial channel interface. In one embodiment, the compressive force regulator may be a flexible flange on the nozzle body, and the flexible flange may have an annular step that includes a surface for engaging a surface of the manifold to limit movement of the flexible flange. The non-flat surface is preferably on the flexible flange and extends from the axial channel to the annular step.